Generative inference model reveals political and economic motivations for road construction in past societies

The construction of roads in past societies reflects a trade-off between a preference for straightness and the influence of slope gradient. This trade-off is, however, also conditioned by other factors, principally the political and economic motivations of a past society. Our understanding of this trade-off and its relationship to these motivations is poor, hindered by qualitative studies that fail to explain variability in road construction, and non-comparative quantitative approaches. Here, we introduce a hierarchical, generative inference model that simulates roads using explicit decision rules to estimate the influence of slope gradient on 62 well-evidenced Roman roads in modern-day Wales. Our results reveal substantial variability, with Roman roads not conforming to a single unifying straightness-gradient trade-off. We relate this trade-off to political and economic motivations to provide a unified, comparative framework for inferring motivations for road construction, both within the Roman Empire and across past societies more broadly.